Liming Enhances Soil Phosphorus Cycling in Long‐Term Agricultural Fields

Liming enhances both organic phosphorus (P) mineralization and the precipitation of inorganic phosphates with calcium (Ca) cations. To better understand how P storage and cycling in soil profiles are regulated by the interaction of long‐term P fertilization and liming, we collected soil samples from three German arable long‐term field experiments in Berlin‐Dahlem (Albic Luvisol; sandy topsoil [0–30 cm], and loamy subsoil [> 30 cm]), Dikopshof (Haplic Luvisol; silty‐loamy topsoil, and clayey‐loamy subsoil), and Thyrow (Albic Luvisol; sandy soil). Treatments within each of these experiments had received mineral fertilization with NKPCa (N: nitrogen; K: potassium; P: phosphorus; Ca: calcium, referring to liming), NKCa, NKP, and NK or no fertilizer application (none) for at least 60 years. Soil P stocks down to 100 cm depth were assessed by Hedley sequential P fractionation and the oxygen isotopic composition of 1 M HCl‐extractable phosphate (δ 18 O P ) was analyzed as an indicator of the degree of microbial P cycling over the decades of experimental duration. We found that mineral P fertilization increased soil total P stocks in all P fractions regardless of differences in soil clay content among the different experiments. Liming significantly decreased NaHCO 3 ‐Pi (Pi: inorganic P) and NaOH‐Pi stocks by up to 50% across the three experiments and soil depths, but tended to increase Po (organic P) stocks in these fractions by up to 40%, reflecting enhanced P uptake into plant and microbial biomass when acidic soil conditions were improved by lime application. Soil HCl‐Pi stocks in treatments with long‐term P fertilization and liming were larger by a factor of up to 1.8 compared to the unfertilized control plots, while especially the plots without P fertilization showed smaller δ 18 O P values of 11‰ in the subsoil. These results indicate that, on the one hand, biological P cycling was enhanced in fertilized treatments, but on the other hand, soluble Pi was precipitated as secondary Ca–P minerals into stable P fractions. These changes occurred both in the topsoil and upper subsoil (30–50 cm). We conclude that the combined application of long‐term P fertilization and liming to the surface soil also increased the utilization of subsoil P.